1. Field of the Invention
The present invention relates to a method and an apparatus for polishing a workpiece, and more particularly to a method and an apparatus for polishing a workpiece such as a semiconductor wafer, a glass substrate, or a liquid crystal panel which is required to be highly cleaned.
2. Description of the Related Art
As semiconductor devices become more highly integrated in recent years, circuit interconnections become finer and the distances between those interconnections also become smaller. Photolithographic processes for producing interconnections that are 0.5 μm wide or smaller, particularly, require a flat image-focusing plane for the stepper because the depth between focal points is small. If a dust particle whose size is greater than the distances between the interconnections is present on a semiconductor substrate, then an undesirable short circuit tends to occur between interconnections through the dust particle.
Therefore, it is important that the workpiece processing must produce a flat and clean workpiece. These processing requirements apply equally to other workpiece materials, such as glass substrates for photo-masking or liquid crystal display panels.
One conventional polishing apparatus is shown in FIG. 9 of the accompanying drawings. As shown in FIG. 9, the conventional polishing apparatus includes a polishing unit 10, a loading/unloading unit 21, a feed robot 22, and two cleaning machines 23a, 23b. As shown in FIG. 10 of the accompanying drawings, the polishing unit 10 comprises a turntable 12 with a polishing cloth 11 attached to an upper surface thereof, and a top ring 13 for holding a workpiece 1 such as a semiconductor wafer and pressing the workpiece 1 against the polishing cloth 11 on the turntable 12.
In operation, the workpiece 1 is supported on the lower surface of the top ring 13, and pressed by a lifting/lowering cylinder against the polishing cloth 11 on the turntable 12 which is being rotated. A polishing solution (abrasive solution) Q is supplied from a polishing solution nozzle 14 onto the polishing cloth 11 and retained by the polishing cloth 11. The lower surface of the workpiece 1 is polished by the polishing cloth 11 while the polishing solution Q is being present between the workpiece 1 and the polishing cloth 11.
The turntable 12 and the top ring 13 rotate at respective speeds that are independent of each other. The top ring 13 holds the workpiece 1 with its edges being spaced distances “a”, “b” from the center and the circumferential edge of the turntable 12. Thus, the entire lower surface of the workpiece 1 is uniformly polished at a high polishing rate. The workpiece 1 has a diameter “d”. The turntable 12 has a diameter “D” which is selected to be at least twice the diameter “d” of the workpiece 1, as indicated by the following equation:D=2(d+a+b)
After having been polished, the workpiece 1 is cleaned in one or more cleaning processes and dried by the cleaning machines 23a, 23b, and then housed in a delivery cassette 24 of the loading/unloading unit 21, when the workpiece 1 is cleaned, it may be scrubbed by a brush of nylon, mohair or the like, or a sponge of PVA (polyvinyl alcohol).
In the conventional polishing apparatus, since the relative displacement between the turntable 12 and the top ring 13 is large, and the relative sliding speed between them is also large, the workpiece 1 can be polished efficiently to a flat finish. However, the polished surface of the workpiece 1 tends to have large surface roughness.
In order to produce a polished workpiece of better surface quality, consideration may be given to using two turntables which are operated by varying the surface roughnesses of the polishing cloths, rotational speeds and types of polishing solutions. However, as mentioned above, the diameter of the turntable is larger than twice that of the workpiece diameter, and each apparatus takes up a large floor space area which leads to higher facility costs. These problems becomes more ignorable as the semiconductor manufacturing industry seeks larger diameter substrates.
While it is possible to use one turntable to produce a superior surface quality by varying the type of polishing solution and lowering the rotational speed of the turntable, it is obvious that such an approach leads not only to a potential increase in the cost of polishing solution but also to inevitable lowering in the production efficiency due to a prolonged polishing operation.
In order to make the workpiece clean, there are some cases where scrubbing process is carried out after the workpiece 1 has been polished using the polishing solution Q. However, such scrubbing process fails to remove submicron particles from the polished surface of the workpiece 1, and is not effective enough to clean the polished workpiece 1 if remaining particles are bonded to the workpiece 1 by large bonding strength.
Further, the conventional polishing apparatus of the type described above has an advantage that the entire surface of the workpiece is polished uniformly, because the elasticity of the polishing cloth 11 moderates the effects of undulation and bowing in the workpiece. However, the workpiece such as a semiconductor wafer is susceptible to edge wear caused by excessive polishing around the peripheral edge of the wafer. Further, in order to polish semiconductor wafers with printed wiring patterns, it is required to obtain a polished surface having flatness of less than 1,000 angstrom by removing any micro-protrusions from uneven surface of the semiconductor wafer. However, the polishing cloth 11 is unable to meet this requirement because the elasticity of the polishing cloth allows the cloth itself to deform and the material from recessed regions as well as from protruding regions is removed.